The need for an apparatus capable of diffusely illuminating a variety of objects and subjects is well known. See for instance U.S. Pat. Nos. 4,335,421 and 4,669,031, respectively disclosing apparati for illuminating x-ray negatives from behind or photographic subjects directly, and discussing other uses such as illuminated tracing tables. All of these applications require either a diffusely illuminated surface (for lighting from behind) or a surface which radiates a diffuse light for direct illumination. A number of apparati have been proposed to suit these applications. See for instance the U.S. patent numbers referenced above.
In the photographic industry, particularly in that branch of the industry dealing with the photography of large subjects for advertising or editorial purposes such as for display quality photographs of new automobiles, an apparatus generally referred to as a banklight or softlight is employed. These banklights are generally large compared to other lighting fixtures, as they are generally in the size range of ten feet wide by thirty feet long, although both larger and smaller versions are also known. Conventional banklights are known to produce an approximation of diffuse light, but they are large, cumbersome, and time consuming in set-up and takedown, and therefore very expensive to operate. One such conventional banklight apparatus in shown generally in cross-section in FIG. 11. The reflective surface of such a banklight may be a parabolic, ellipsoidal, or circular curve, and the light source schematically shown is typically a line of photo flashtubes or quartz halogen lamps. The diffusing fabric attempts to make up for the nonuniformity of the reflected light rays coming from the reflecting surface, but is only partially successful in doing so.
Intensity of a light source is normally measured with an incident light meter, a meter made to average the amount of light which is striking the meter at any given moment. In the case of a banklight, which is a diffused light source, the amount of light radiating from it is a function of how much light is inside the banklight, factored by the efficiency of the reflector system and the degree to which light is diffused as it passes through the diffusion medium. The outer or primary diffuser is usually considered the front of the banklight.
When discussing light output or intensity from a banklight, it will be useful to limit these terms to the amount of light passing through the outer or primary diffuser, and to speak in terms of the particular light output at any one point on the diffuser, say from a one square inch patch. Such output could be measured with an incident light meter, especially if equipped with a flat light collector, and if held on the spot on the diffuser to be measured. However, the use of a reflective spot meter will be both more convenient and more precise. This type of meter measures the amount of light being reflected off an object. For a banklight, it is appropriate to consider the light coming through a diffuser, especially the primary diffuser, the same as the light reflecting off a white card. Reflective spot meters are optical instruments which require that one look through the lens, and they typically measure the light reflected from a small area, equal to about 1 degree of the field of view. Use of a spot meter has an additional benefit: the spot meter "sees" the diffusion just as a human eye would. Most available diffusion materials are woven fabrics, and to some extent (though calendarization can ameliorate this effect) one can "see" through the weave to the undiffused light within. Since the apparent surface intensity is thus affected to a small extent by the intensity of what is behind the diffuser, the use of a spot meter can provide a more precise estimate of actual available light, and the quality of that light.
Treating the diffuser as if it were a white object reflecting light is important. Banklights not only provide the incident light which illuminates the object being photographed; in the case of automobiles and other highly reflective objects, the diffusion area of the banklight is reflected in the car, and is readily visible in the highlight areas of the object. Therefore, to have the greatest control of the final look of the object in the photograph, it is extremely important to have a banklight with the capability of being gradated, or even variegated and gradated, to maximize the range of possible tonal qualities present in the object in the photograph, without having stark (ungradated) intensity boundaries show up as reflection aberrations in the object in the photograph.
None of the known apparati are capable of quick and easy set-up and take down and none of them are well adapted for banklight configurations as large as eighteen by forty-eight feet, or even larger. In addition none of the known systems employ multiple lines of light sources, each line independently controlled so as to produce, when desired, a gradual and uniform variation of light intensity across the surface of the light fixture, and no known apparatus employs a system of reflecting and diffusing panels to achieve, as desired, either a virtually uniform diffusion of light, or a controlled gradation of diffuse light, while at the same time achieving a very low ratio of thickness of the apparatus to its width. No known banklights employ a plurality of individually controlled light sources and gradation cells to selectably achieve uniform diffusion or intensity/color gradations across the banklight, or variegated color/intensity patterns with gradated boundary zones at the primary diffuser.